To evaluate the potential risks associated with space exploration, the proposed studies will assess the impact of altered gravity on the developing CNS. Specifically, these studies will probe the hypothesis that changes in gravity affect cerebellar development and that this effect is mediated by altered thyroid status. This hypothesis is based on the following: (1) both the CNS and thyroid status are inhibited in astronauts and in adult animals exposed to micro- and hypergravity; (2) thyroid hormone (TH) is critical for normal CNS development; and (3) motor coordination, controlled by the cerebellum, is affected in both astronauts and animals under altered gravity. The experiments designed to test this hypothesis will utilize the hypergravity paradigm of a 24-ft centrifuge, since it provides the most adaptable system for studying the effect of altered gravity on developing mammals. Based on preliminary findings suggesting that exposure to centrifugation from gestation through weaning results in changes in the developing cerebellum and altered thyroid status in rat neonates, we predict that: (1) changes in cerebellar size and structure observed at 15G will become more pronounced at higher gravitational loading; (2) the mild hypothyroidism observed at 1.5 G will become more severe at higher gravitational loading. The effect of hypergravity on neonatal cerebellum will be evaluated primarily in terms of the celebellar size and number of Purkinje and granule cells; the thyroid status of dams and neonates will be assessed primarily in terms of plasma thyroid stimulating hormone (TSH), free T3 and T4, and neonatal cerebellar T3 and T4 at P6 to P21. These parameters will be compared between stationary controls (SC), rotational controls (RC) and hypergravity-exposed (HG, 1.5G. 1.65G. and 1.75G) dams and/or pups during one of the three developmental periods: (1) the second part of embryonic development through the neonatal period (Gil to P21); (2) the second part of embryonic development only (Gil-P 1); and (3) the neonatal period only (P1 to P21). To examine the possible contribution of other factors such as malnutrition, pair-fed controls will be included to evaluate the contribution of maternal-offspring interactions, HG neonates will be cross-fostered to SC dams. The status of the hypothalamic-pituitary-thyroid (HPT) axis will be examined by measuring neonatal serum thyroid-binding proteins (TBG, TTR) and thyroid size. Should these studies support the direct involvement of THin the neonatal response to hypergravity, a TI! supplement will be given to dams or pups to attempt to prevent the adverse effects of altered gravity. Additional molecular studies, including immuno-chemical immunohistological, and gene expression analyses (northern blots, RPA, and rat-specific cDNA arrays, focusing on TH-regulated genes) will probe the molecular mechanism(s) involved in gravity response. These studies will aid in developing an understanding of how altered gravity affects CNS development, what role the thyroid hormone plays in that response, and whether this response can be modulated by hormonal therapy.